leftToMaybe/rightToMaybe

[Erik Hesselink]

> Ivan Lazar Miljenovic wrote:
>> If we're starting to add functions to Data.Either, I've often used
>> something like this:
>>
>> mapEither :: (l -> l') -> (r -> r') -> Either l r -> Either l' r'
>> mapEither fl fr = either (Left . fl) (Right . fr)

This function is hidden in Control.Arrow as (+++).

There are more useful functions in there, but it's sometimes hard to
find them, especially now that we can no longer use type operators for
type variables.

Erik